/etc/wide-dhcpv6/dhcp6c-ifupdown is in wide-dhcpv6-client 20080615-16.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | #!/bin/sh
# Updates information whenever a network interface is brought up.
[ "$IFACE" = "lo" ] && exit 0
[ -r /etc/default/wide-dhcpv6-client ] || exit 0
[ -x /usr/sbin/dhcp6ctl ] || exit 0
# Check if dhcp6c is running
pidof dhcp6c > /dev/null 2>&1 ; [ $? -eq 1 ] && exit 0
. /etc/default/wide-dhcpv6-client
case $MODE in
    start|stop)
        for i in $INTERFACES ; do
            if [ "$IFACE" = "$i" ] ; then
                /usr/sbin/dhcp6ctl $MODE interface $IFACE
                exit 0
            fi
        done
    ;;
    *)
        if [ "$VERBOSITY" = "1" ] ; then
            echo "dhcp6c-ifupdown: unknown mode \"$MODE\""
        fi
        exit 1
    ;;
esac
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